Preventing harmful polarization of solar cells

ABSTRACT

In one embodiment, harmful solar cell polarization is prevented or minimized by providing a conductive path that bleeds charge from a front side of a solar cell to the bulk of a wafer. The conductive path may include patterned holes in a dielectric passivation layer, a conductive anti-reflective coating, or layers of conductive material formed on the top or bottom surface of an anti-reflective coating, for example. Harmful solar cell polarization may also be prevented by biasing a region of a solar cell module on the front side of the solar cell.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/477,796, filed on Jun. 3, 2009 now U.S. Pat. No. 7,786,375, which isa continuation of U.S. application Ser. No. 11/210,213, filed on Aug.22, 2005, now U.S. Pat. No. 7,554,031, which claims the benefit of U.S.Provisional Application No. 60/658,706, filed Mar. 3, 2005, all of whichare incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to solar cells, and moreparticularly but not exclusively to solar cell structures, modules,fabrication, and field installation.

2. Description of the Background Art

Solar cells are well known devices for converting solar radiation toelectrical energy. They may be fabricated on a semiconductor wafer usingsemiconductor processing technology. Generally speaking, a solar cellmay be fabricated by forming p-type regions and n-type regions in asilicon substrate. Each adjacent p-type region and n-type region forms ap-n junction. Solar radiation impinging on the solar cell createselectrons and holes that migrate to the p-type and n-type regions,thereby creating voltage differentials across the p-n junctions. In abackside contact solar cell, the p-type and n-type regions are coupledto metal contacts on the backside of the solar cell to allow an externalelectrical circuit or device to be coupled to and be powered by thesolar cell. Backside contact solar cells are also disclosed in U.S. Pat.Nos. 5,053,083 and 4,927,770, which are both incorporated herein byreference in their entirety.

Several solar cells may be connected together to form a solar cellarray. The solar cell array may be packaged into a solar cell module,which includes protection layers to allow the solar cell array towithstand environmental conditions and be used in the field. Ifprecautions are not taken, solar cells may become highly polarized inthe field, causing reduced output power. Techniques for preventingharmful polarization of solar cells are disclosed herein.

SUMMARY

In one embodiment, harmful solar cell polarization is prevented orminimized by providing a conductive path that bleeds charge from a frontside of a solar cell to the bulk of a wafer. The conductive path mayinclude patterned holes in a dielectric passivation layer, a conductiveanti-reflective coating, or layers of conductive material formed on thetop or bottom surface of an anti-reflective coating, for example.Harmful solar cell polarization may also be prevented by biasing aregion of a solar cell module on the front side of the solar cell.

These and other features of the present invention will be readilyapparent to persons of ordinary skill in the art upon reading theentirety of this disclosure, which includes the accompanying drawingsand claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exploded view of an example solar cell module that maytake advantage of embodiments of the present invention.

FIG. 2 schematically shows a cross-section of the solar cell module ofFIG. 1.

FIGS. 3A and 3B show models for the mechanism the inventors believecauses solar cell polarization.

FIGS. 4A, 4B, 5A, 5B, 5C, 5D, and 6 schematically show cross sections ofsolar cells in accordance with embodiments of the present invention.

FIG. 7A schematically shows a solar cell module in accordance with anembodiment of the present invention.

FIGS. 7B and 7C schematically show solar energy systems in accordancewith embodiments of the present invention.

The use of the same reference label in different drawings indicates thesame or like components. Drawings are not necessarily to scale unlessotherwise noted.

DETAILED DESCRIPTION

In the present disclosure, numerous specific details are provided, suchas examples of apparatus, components, and methods, to provide a thoroughunderstanding of embodiments of the invention. Persons of ordinary skillin the art will recognize, however, that the invention can be practicedwithout one or more of the specific details. In other instances,well-known details are not shown or described to avoid obscuring aspectsof the invention.

Referring now to FIG. 1, there is shown an exploded view of an examplesolar cell module 100 that may take advantage of embodiments of thepresent invention. Such a solar cell module is also disclosed incommonly-assigned U.S. application Ser. No. 10/633,188, filed on Aug. 1,2003. It is to be noted, however, that embodiments of the presentinvention are also applicable to other solar cell modules.

In the example of FIG. 1, the solar cell module 100 includes atransparent cover 104, encapsulants 103 (i.e., 103-1, 103-2), a solarcell array 110 comprising interconnected solar cells 200, and a backsheet 102. The solar cell module 100 is a so-called “terrestrial solarcell module” in that it is typically used in stationary applications,such as on rooftops or by power generating stations. As such, the solarcell module 100 is installed with the transparent cover 104 facing thesun. In one embodiment, the transparent cover 104 comprises glass. Thefront sides of the solar cells 200 face towards the sun by way of thetransparent cover 104. Encapsulants 103 crosslink and bond the solarcells 200, the cover 104, and the back sheet 102 to form a protectivepackage. In one embodiment, the encapsulants 103 comprisepoly-ethyl-vinyl acetate (“EVA”).

The backsides of the solar cells 200 face the back sheet 102, which isattached to the encapsulant 103-1. In one embodiment, the back sheet 102comprises Tedlar/Polyester/EVA (“TPE”) from the Madico company. In theTPE, the Tedlar is the outermost layer that protects against theenvironment, the polyester provides additional electrical isolation, andthe EVA is a non-crosslinked thin layer that promotes adhesion to theencapsulant 103-1. Alternatives to TPE for use as the back sheet 102include Tedlar/Polyester/Tedlar (“TPT”), for example.

FIG. 2 schematically shows a cross-section of the solar cell module 100.FIG. 2 has been annotated with example materials for ease ofunderstanding. However, it is to be noted that other materials may alsobe employed without detracting from the merits of the present invention.For purposes of the present disclosure, the front side of the solar cellcomprises materials, components, and features on the front side of thewafer 203 (i.e. from the passivation layer 202 towards the cover 104),while the backside of the solar cell comprises those on the backside ofthe wafer 203 (i.e. from the doped regions 204 towards the back sheet102). The materials on the front side of the solar cell 200 areconfigured to face the sun during normal operation. The materials on thefront side of the solar cell 200 are transparent by nature or thicknessto allow solar radiation to shine through.

In the example of FIG. 2, a wafer 203 comprises an n-type silicon waferwith an n-type front side diffusion region 207. Front side diffusionregion 207 has been schematically separated with a dash line to indicatethat it is in the silicon of wafer 203. A dielectric passivation layer202, which comprises silicon dioxide in the example of FIG. 2, is formedon the front side of the wafer 203. An anti-reflective coating (“ARC”)201 is formed on top of the dielectric passivation layer 202. In oneembodiment, the anti-reflective coating 201 comprises silicon nitrideformed to a thickness of about 400 Angstroms by plasma enhanced chemicalvapor deposition (PECVD). In one embodiment, the passivation layer 202comprises silicon dioxide formed to a thickness of about 200 Angstroms.The passivation layer 202 may be grown directly on the top surface ofthe wafer 203 by high temperature oxidation.

In the example of FIG. 2, p-type doped (“P+”) and n-type doped (“N+”)regions 204 serving as charge collection junctions of the solar cell 200are formed in the wafer 203. The p-type and n-type doped regions 204 mayalso be formed external to the wafer 203, such as in a layer formed onthe backside of the wafer 203, without detracting from the merits of thepresent invention. Metal contacts 206 are formed on the backside of thesolar cell 200, with each metal contact 206 being coupled to acorresponding p-type doped or n-type doped collection region. An oxidelayer 205 is patterned to allow metal contacts 206 to be connected tothe doped regions 204. Typically, metal contacts 206 are connected tometal contacts of other solar cells 200 in the solar cell array 110.Metal contacts 206 allow an external circuit or device to receiveelectrical current from the solar cell module 100. The solar cell 200 isa backside contact solar cell in that all electrical connections to itscollection regions are formed on its backside.

As shown in FIG. 2, the solar cell 200 is protected by back sheet 102,encapsulants 103, and cover 104. A frame 211 surrounds the solar cell200 and its protection layers. Under certain conditions, the outputpower generation capability of the solar cell module 100 may besubstantially reduced. This reduction in output power is reversible inthat the solar cell module 100 may be restored back to its originalcondition by, for example, biasing the solar cell module 100 with highvoltage in a beneficial current flow direction. The inventors believethat this output power reduction is due to the solar cell 200 becomingpolarized when charge leaks from the front side of the solar cell 200 tothe frame 211 as indicated by arrow 212. In one example, positive chargecarriers leak from the front side of the solar cell 200, thereby leavingthe surface of the anti-reflective coating 201 negatively charged. Thenegative charge on the surface of the anti-reflective coating 201attracts positively charged light generated holes, some of whichrecombine with electrons in the n-type silicon wafer 203 instead ofbeing collected at a doped collection region.

Because the solar cell 200 has an n-type front side diffusion region,harmful polarization may occur when, in the field, the dielectricpassivation layer 202 has an electric field polarity such that electronsare repelled, and holes attracted, to the interface between thedielectric passivation layer 202 and front side diffusion region 207,i.e., when the potential of the dielectric passivation layer 202 is lessthan the front side diffusion region 207. In field operation, this wouldoccur when the solar cell 200 is operated at a positive voltage withrespect to ground. In other embodiments where a solar cell has a p-typefront side diffusion region, harmful solar cell polarization may occurwhen the solar cell becomes negatively biased (i.e. becomes morenegative) relative to ground in the field. As is well-known, a p-typesilicon wafer may be doped to have an n-type front side diffusionregion. Similarly, an n-type silicon wafer may be doped to have a p-typefront side diffusion region. Although the example solar cell 200 has ann-type front side diffusion region in an n-type silicon wafer, theteachings of the present invention may be adapted to other types ofsolar cell substrates.

FIG. 3A schematically shows a model for the mechanism that the inventorsbelieve is responsible for solar cell polarization. In the model of FIG.3A, current flows to or from the solar cell through the front of theglass (e.g. cover 104) and is leaked off by a shunt to the back surfaceof the solar cell. Resistance R_(gl) represents the leakage resistancefrom the nitride ARC (e.g. anti-reflective coating 201) to the glassfront and R_(sh) is the shunt leakage from the nitride ARC to the backof the solar cell. In reality, there will be a distributed voltagedeveloped across the solar cell which starts at a low value at the edgeand builds up toward the middle. In any case, the nitride ARC to siliconwafer voltage shouldn't exceed the oxide breakdown voltage. In FIGS. 3Aand 3B, the capacitance “C” represents a capacitor comprising an oxidepassivation layer (e.g. dielectric passivation layer 202) serving as adielectric, the nitride ARC serving as a first capacitor plate, and thesilicon wafer serving as a second capacitor plate.

FIG. 3B schematically shows the lumped element approximation equivalentcircuit for the structure of FIG. 3A. For purposes of this analysis, thevoltages are referenced to the back of the solar cell. The transientsolution to this circuit, assuming that the starting gate voltage iszero is shown by equation EQ. 1.

$\begin{matrix}{{V_{G}(t)} = {V\frac{R_{sh}}{R_{sh} + R_{gl}}\left( {1 - {\mathbb{e}}^{{- t}/\tau}} \right)}} & {{EQ}.\mspace{14mu} 1}\end{matrix}$Where

$\tau = {\frac{{CR}_{sh}R_{gl}}{R_{sh} + R_{gl}} = {CR}_{eq}}$and R_(eq) is the parallel equivalent resistance. V_(G) represents thevoltage on the front EVA encapsulant, which behaves like a gate of ametal oxide semiconductor (MOS) transistor. The gate oxide of the MOStransistor is the oxide dielectric passivation layer. As mentioned, thecapacitance “C” represents the capacitor formed by the nitride ARC, theoxide passivation layer, and the silicon wafer.

Upon power up of the solar cell, the gate (i.e. front side EVAencapsulant) will ramp upward and reach a voltage V_(T) which causes acertain degradation amount after a degradation time t_(deg) representedby the equation EQ. 2.

$\begin{matrix}{t_{\deg} = {{\tau ln}\left\{ \frac{1}{1 - {\frac{V}{V_{T}}\left( {1 + \frac{R_{gl}}{R_{sh}}} \right)}} \right\}}} & {{EQ}.\mspace{14mu} 2}\end{matrix}$

In equation EQ. 2, it is assumed that “V” is positive, but is also truefor negative V and negative V_(T) (threshold voltage of the MOStransistor) if absolute values for voltages are used. For the usual casewhen

${{V\frac{R_{sh}}{R_{gl} + R_{sh}}}\operatorname{>>}V_{T}},$equation EQ. 2 reduces to equation EQ. 3.

$\begin{matrix}{t_{\deg} = {{CR}_{gl}\frac{V_{T}}{V}}} & {{EQ}.\mspace{14mu} 3}\end{matrix}$From equation EQ. 3, it can be readily seen that for high voltages, thetime to a specific amount of degradation is inversely proportional tothe applied voltage.

The recovery of the gate voltage for zero applied voltage is given byEQ. 4V _(G)(t)=V _(G)(0)e ^(−t/τ)  EQ. 4If V_(T) is the threshold where negligible degradation occurs, then therecovery time t_(rec) is given by equation EQ. 5.

$\begin{matrix}{t_{rec} = {{\tau ln}\left( \frac{V_{G}(0)}{V_{T}} \right)}} & {{EQ}.\mspace{14mu} 5}\end{matrix}$

Ultra violet rays will have the effect of adding an additional shuntresistance in parallel with the existing one. This can be seen byassuming that the rate which ultra violet injects electrons from thenitride ARC to the silicon wafer is proportional to the trapped electrondensity. But the voltage across the capacitor “C” (see FIG. 3B) isproportional to the trapped charge, therefore the current isproportional to the voltage on the gate capacitor; i.e.,resistance-like. Assuming that this resistance is small compared to theother shunts (which it must be in order to have an effect) then therecovery time in the light t_(rec, light) is given by equation EQ. 6.

$\begin{matrix}{t_{{rec},{light}} = {R_{{sh},{light}}C\;{\ln\left( \frac{V_{G}(0)}{V_{T}} \right)}}} & {{EQ}.\mspace{14mu} 6}\end{matrix}$

The conditions necessary for the ultra violet induced shunt to besufficient to keep the solar cell module from degrading may becalculated. This requires the condition given by EQ. 7 to be satisfied.

$\begin{matrix}{{V\frac{R_{{sh},{light}}}{R_{gl}}} < V_{T}} & {{EQ}.\mspace{14mu} 7}\end{matrix}$The above equations can be rearranged to show that EQ. 7 is satisfiedwhen the recovery time in the light is given by equation EQ. 8.

$\begin{matrix}{{t_{{rec},{light}} < t_{\deg,{dark}}} = {{CR}_{gl}\frac{V_{T}}{V}}} & {{EQ}.\mspace{14mu} 8}\end{matrix}$In other words, if the module solar cell module recovers in sunlight,when unbiased, in a shorter time than it takes to degrade in the darkwith an applied bias, then the module will be stable in sunlight withthat applied bias.

In some embodiments, harmful solar cell polarization is prevented orminimized by increasing vertical electrical conductivity in the frontside anti-reflective coating/passivation layer stack. In theseembodiments, charge is bled from the front side of the solar cell to thebulk of the wafer. These embodiments are now described with reference toFIGS. 4A and 4B.

FIG. 4A schematically shows a cross section of a solar cell 200A inaccordance with an embodiment of the present invention. The solar cell200A is a specific embodiment of the solar cell 200 shown in FIG. 2. Thesolar cell 200A is the same as the solar cell 200 except for the use ofa very thin oxide (i.e. silicon dioxide) layer 202A as a passivationlayer 202 and an anti-reflective coating 201A as an anti-reflectivecoating 201. In the example of FIG. 4A, the anti-reflective coating 201Amay comprise silicon carbide having a thickness of about 400 Angstromsand the wafer 203 comprises an N-type silicon wafer. The thin oxidelayer 202A is preferably thin enough to bleed charge to the bulk of thewafer, to prevent charge buildup, and such that oxide breakdown occurswhen it develops a relatively high voltage. The thin oxide layer 202Amay be formed directly on the wafer 203. In one embodiment, the thinoxide layer 202A is formed to a thickness of about 10 Angstroms to 20Angstroms using an ozone oxide process, which involves dipping the wafer203 in a bath comprising ozone suspended in deionized water.

FIG. 4B schematically shows a cross section of a solar cell 200B inaccordance with an embodiment of the present invention. The solar cell200B is a specific embodiment of the solar cell 200 shown in FIG. 2. Thesolar cell 200B is the same as the solar cell 200 except for the use ofa patterned dielectric passivation layer 202B as a passivation level202. In the example of FIG. 4B, passivation layer 202B comprises silicondioxide, the anti-reflective coating 201 comprises silicon nitride, andthe wafer 203 comprises an N-type silicon wafer. As shown in FIG. 4B,the passivation layer 202B has been patterned to have holes that allow asilicon nitride anti-reflective coating 201 to contact the silicon wafer203. This allows charge on the anti-reflective coating 201 to bleed tothe bulk of the wafer 203 through the patterned holes in the oxidepassivation layer 202B. Each hole in passivation layer 202B may beformed using a conventional lithography process, and be as small as theavailable lithography equipment allows. The patterned holes may beseparated by about 0.1 mm to about 2.0 mm from each other, for example.The perforated passivation layer 202B advantageously prevents solar cellpolarization by preventing charge build up in the anti-reflectivecoating 201.

In some embodiments, lateral conduction on the front side and towardsthe edges of the solar cell is increased to prevent solar cellpolarization. Because passivation layers have natural defects (i.e.naturally formed holes) through them, it is possible for a conductiveanti-reflective coating to bleed accumulated charge to the bulk of thewafer through the defects. However, some solar cell anti-reflectivecoatings may not be conductive enough for this to occur. Accordingly, insome embodiments, a conductive layer is formed laterally to contact theanti-reflective coating to allow charge to bleed from theanti-reflective coating to the bulk of the wafer by way of theconductive layer and the natural defects in the passivation layer. Inother embodiments, the anti-reflective coating itself is sufficientlyconductive. These embodiments are now described with reference to FIGS.5A-5D.

FIG. 5A schematically shows a cross section of a solar cell 200C inaccordance with an embodiment of the present invention. The solar cell200C is a specific embodiment of the solar cell 200 shown in FIG. 2. Thesolar cell 200C is the same as the solar cell 200 except that atransparent conductive coating 501 is formed on a top surface of theanti-reflective coating 201. In the example of FIG. 5A, the passivationlayer 202 comprises silicon dioxide, the anti-reflective coating 201comprises silicon nitride, and the wafer 203 comprises an N-type siliconwafer. In one embodiment, the transparent conductive coating 501comprises a conductive organic coating, such as the PEDOT/PSS(Baytron-P) coating. The transparent conductive coating 501 may besprayed or screen-printed directly on top of the anti-reflective coating201. The transparent conductive coating 501 may be formed to a thicknessof about 100 Angstroms, for example. The transparent conductive coating501 may be applied on the solar cell 200 as a last step in the solarcell fabrication process, just before encapsulation.

Because the silicon nitride anti-reflective coating 201 is notsufficiently conductive, charge in the silicon nitride can only travel ashort distance, which is not enough to reach natural defects in thepassivation layer 202. The transparent conductive coating 501 allowscharge in the anti-reflective coating 201 to travel a distancesufficient to reach natural defects in the passivation layer 202 andbleed to the bulk of the wafer 203.

FIG. 5B schematically shows a cross section of a solar cell 200D inaccordance with an embodiment of the present invention. The solar cell200D is a specific embodiment of the solar cell 200 shown in FIG. 2. Thesolar cell 200D is the same as the solar cell 200 except that aconductive anti-reflective coating (ARC) 201B is used as ananti-reflective coating 201. In the example of FIG. 5B, the passivationlayer 202 comprises silicon dioxide and the wafer 203 comprises anN-type silicon wafer. The conductive ARC 201B advantageously minimizessolar cell polarization by preventing charge from accumulating in it.Charge in the conductive ARC 201B may bleed to the bulk of the wafer byway of natural defects in the passivation layer 202.

In one embodiment, the conductive ARC 201B comprises a naturallyconductive (i.e. conductive without addition of impurities)anti-reflective coating, such as titanium oxide (TiO₂).

In other embodiments, the conductive ARC 201B comprises a non-conductiveanti-reflective material that is made conductive by addition ofimpurities. One way of doing so is by adding metal impurities from ametal gas source during formation of the anti-reflective material on thepassivation layer 202. For example, the conductive ARC 201B may comprisetin oxide doped with fluorine (SnO:F), zinc oxide doped with boron(ZnO:B), or silicon carbide doped with phosphorus (SiC:P) or boron(SiC:B). As a specific example, the conductive ARC 201B may be formed toa thickness of about 400 Angstroms by plasma enhanced chemical vapordeposition (PECVD) of silicon carbide (SiC) with the addition ofphosphine gas (PH₃) or diborane gas (B₂H₆) during deposition.

FIG. 5C schematically shows a cross section of a solar cell 200E inaccordance with an embodiment of the present invention. The solar cell200E is a specific embodiment of the solar cell 200 shown in FIG. 2. Thesolar cell 200E is the same as the solar cell 200 except that atransparent conductive layer 502 is formed on top of the anti-reflectivecoating 201. In the example of FIG. 5C, the passivation layer 202comprises silicon dioxide, the anti-reflective coating 201 comprisessilicon nitride, and the silicon wafer 203 comprises an N-type wafer.Like the conductive coating 501 of solar cell 200C (FIG. 5A), thetransparent conductive layer 502 allows charge in the anti-reflectivecoating 201 to travel a distance sufficient to reach natural defects inthe passivation layer 202 and bleed to the bulk of the wafer 203.

Transparent conductive layer 502 may be evaporated, sputtered, ordeposited directly on top of the anti-reflective coating 201. Thetransparent conductive layer 502 may comprise a transparent conductiveoxide, such as tin oxide doped with fluorine (SnO:F), zinc oxide dopedwith boron (ZnO:B), or silicon carbide doped with phosphorus (SiC:P) orboron (SiC:B) formed to a thickness of about 200 Angstroms.

FIG. 5D schematically shows a cross section of a solar cell 200F inaccordance with an embodiment of the present invention. The solar cell200F is a specific embodiment of the solar cell 200 shown in FIG. 2. Thesolar cell 200F is the same as the solar cell 200 except that arelatively thin (e.g. about 200 Angstroms) conductive layer 503 isformed between the passivation layer 202 and the anti-reflective coating201. In the example of FIG. 5D, the passivation layer 202 comprisessilicon dioxide, the anti-reflective coating 201 comprises siliconnitride, and the silicon wafer 203 comprises an N-type wafer. The thinconductive layer 503 allows charge to bleed from the anti-reflectivecoating 201, to the thin conductive layer 503, and to the bulk of thewafer 203 through natural defects in the passivation layer 202. In oneembodiment, the conductive layer 503 comprises polysilicon formed to athickness of about 200 Angstroms directly on the top surface of thepassivation layer 202. The anti-reflective coating 201 may be formeddirectly on a surface of the conductive layer 503. The conductive layer503 may be formed by PECVD and in-situ (i.e. in the same chamber orcluster tool in one loading) with the formation of the anti-reflectivecoating 201. The conductive layer 503 may also comprise tin oxide dopedwith fluorine (SnO:F), zinc oxide doped with boron (ZnO:B), or siliconcarbide doped with phosphorus (SiC:P) or boron (SiC:B) formed to athickness of about 200 Angstroms.

In the embodiments of FIGS. 4 and 5, conductivity from the front side ofthe solar cell to the bulk of the wafer is increased to prevent harmfulsolar cell polarization. This is equivalent to lowering the shuntresistance R_(sh) in the model of FIG. 3B. In other embodiments, theresistance from the front side of the solar cell to the rest of themodule by way of the transparent cover is increased to prevent chargeleakage. This is equivalent to increasing the resistance R_(gl) in themodel of FIG. 3B. Resistance from the front side of the solar cell tothe rest of the solar cell module may be increased by blocking thecharge leakage path, as now described with reference to FIG. 6.

FIG. 6 schematically shows a cross section of a solar cell 200G inaccordance with an embodiment of the present invention. The solar cell200G is a specific embodiment of the solar cell 200 shown in FIG. 2. Thesolar cell 200G is the same as the solar cell 200 except that atransparent electrical insulator layer 691 is formed over theanti-reflective coating 201. In the example of FIG. 6, the passivationlayer 202 comprises silicon dioxide, the anti-reflective coating 201comprises silicon nitride, and the silicon wafer 203 comprises an N-typewafer. The electrical insulator layer 691 is formed over theanti-reflective coating 201 to prevent solar cell polarization bypreventing charge from leaking out from the front side of the solar cell200G towards the cover 104 (see FIG. 2). In one embodiment, theelectrical insulator layer 691 comprises silicon dioxide (SiO₂) formedto a thickness of about 0.1 to 1.0 μm by atmospheric pressure chemicalvapor deposition (APCVD).

Harmful solar cell polarization may also be prevented by biasing aregion of a solar cell module on the front side of the solar cell, asnow discussed with reference to FIG. 7A.

FIG. 7A schematically shows a solar cell module 100A in accordance withan embodiment of the present invention. The solar cell module 100A is aspecific embodiment of the solar cell module 100 shown in FIG. 2.Several solar cells 200, along with their interconnects 200, are shownin FIG. 7A. An interconnect 682 serially connects one solar cell 200 toanother. The solar cell module 100A is essentially the same as the solarcell module 100 except that an electrically conductive path is added tobring up the potential of the part of the module in front of the cell toprevent harmful leakage current (i.e., above, at, or within 30V forn-type cell modules). In one embodiment, the conductive path is formedby placing a transparent electrically conductive layer 684 on the backsurface of the transparent cover 104 (e.g. glass) and connecting theconductive layer 684 to the back surface of a solar cell 200. In theexample of FIG. 7A, the conductive layer 684 is electrically connectedto an interconnect 682, which is connected to the backside of a solarcell 200 by way of an electrical connection 683. In the example of FIG.7A, the preferred embodiment is for the conductive layer 684 to beconnected to the interconnect 682 that is connected to the highest (i.e.most positive) or near highest potential solar cell 200 in the array forcells with an n-type front side diffusion region, and the lowest (i.e.,most negative) or near most negative potential solar cells 200 in thearray for cells with a p-type front side diffusion region. Theconductive layer 684 is isolated from the frame of the solar cell module100A to prevent an unsafe condition where a high-voltage is on theexterior of the module. The conductive layer 684 may comprise tin oxidedoped with fluorine (SnO:F), indium tin oxide (ITO), zinc oxide (ZnO),or other transparent oxides or transparent organic conductors. In thepreferred embodiment, this conductive layer has a sheet resistance ofapproximately 5e4 ohm/square. The back sheet 102 is formed on the bottomsurface of the encapsulant 103 as before. In an alternative embodiment,the encapsulant 103 is made electrically conductive to form anear-equipotential field above the solar cells 200; the encapsulant atthe edges of the module remains electrically insulating to prevent anunsafe condition where a high-voltage is on the exterior of the module.

In a system level approach, the entire solar energy system is taken intoconsideration to prevent charge from leaking from the front side of thesolar cell. For example, an array of solar cell modules may be biasedsuch that leaking of charge carriers from the front side of the solarcells is prevented. Example system level approaches to the solar cellpolarization problem are now described with reference to FIGS. 7B and7C.

FIG. 7B schematically illustrates a solar energy system 790 inaccordance with an embodiment of the present invention. In the exampleof FIG. 7B, a solar cell module array 630 has several solar cell modulescomprising inter-connected solar cells 200. The positive output terminalof the solar cell module array 630 is labeled as node 616, while itsnegative output terminal is labeled as node 617. In the example of FIG.7B, the solar cells 200 are series connected such that their positiveterminals are toward the node 616 and their negative terminals aretoward the node 617. There may be other series connected solar cells 200in parallel to the series shown in FIG. 7B.

In the example of FIG. 7B, the solar cell module array 630 is coupled toan inverter 600. An inverter converts direct current (DC) to alternatingcurrent (AC). In the solar energy system 790, the inverter 600 receivesdirect current from the solar cell module array 630 and outputsalternating current to a power grid. As shown in FIG. 7B, a DC to DCconverter 601 converts direct current from the solar cell module array630 to another direct current. The direct current output of the DC to DCconverter 601 is converted to alternating current by DC to AC converter602. The alternating current output of the DC to AC converter 602 isprovided to the power grid by way of an isolation circuit 603.Alternatively, the isolation circuit 603 may be in series between the DCto DC converter 601 and the DC to AC converter 602.

In the solar energy system 790, the positive terminal of the solar cellarray module 630 is grounded. Systems similar to the solar energy system790 may be used in North America and Japan among other countries. Theframe 614, which represents the frame of all solar cell modules in thesolar cell module array 630 is also grounded as indicated by the label611. Grounding the positive terminal of the solar cell module array 630and the frame 614 reduces the potential between the solar cells 200 andthe frame 614, minimizing leakage from the front side of the solar cells200. The positive terminal of the solar cell module array 630 may betied to ground within or outside of the inverter 600.

In the example of FIG. 7B, each solar cell 200 has an n-type front sidediffusion region. In this case, harmful solar cell polarization occursbecause the solar cells 200 become positively biased relative to ground.To prevent harmful polarization, the highest or near highest potentialof the solar cell module array 630 (node 616 in this case) isaccordingly tied to ground. In other embodiments where the solar cellshave a p-type front side diffusion region, harmful polarization mayoccur when the solar cells become negatively biased relative to ground.In that case, the lowest or near lowest potential solar cell in thearray (e.g. the negative output terminal of the solar cell module array)may be tied to ground to prevent harmful solar cell polarization.

FIG. 7C schematically illustrates a solar energy system 795 inaccordance with an embodiment of the present invention. In the exampleof FIG. 7C, the solar cell module array 630 has several solar cellmodules comprising several inter-connected solar cells 200. The positiveoutput terminal of the solar cell module array 630 is labeled as node616, while its negative output terminal is labeled as node 617. In theexample of FIG. 7C, the solar cells 200 are series connected such thattheir positive terminals are toward the node 616 and their negativeterminals are toward the node 617. There may be other series connectedsolar cells 200 in parallel to the series shown in FIG. 7C.

In the example of FIG. 7C, the solar cell module array 630 is coupled toan inverter 650. The inverter 650 receives direct current from the solarcell module array 630 and outputs alternating current to the power grid.As shown in FIG. 7C, a DC to DC converter 651 converts direct currentfrom the solar cell module array 630 to another direct current. Thedirect current output of the DC to DC converter 651 is coupled to a DCto AC converter 652 by an isolation circuit 653. The alternating currentoutput of the DC to AC converter 652 is provided to the power grid.Alternatively, the isolation circuit 653 may be located at the output ofthe DC to AC converter 652 to provide AC output to the power grid.Systems similar to the solar energy system 795 may be employed incountries covered by IEC regulations, such as most European countries,the United Kingdom, and others.

In the example of FIG. 7C, the output of the solar cell array module 630is balanced to +/−½ (i.e. plus/minus half) the value of the totalvoltage of the solar cell module array 630. That is, the voltage at node616 is ideally +½ of the total voltage of the solar cell module array630, while the voltage at node 617 is ideally −½ of the total voltage ofthe solar cell array module 630. The resistors 672 and 673 are highvalue resistors (or varistors) that balance the output of the solar cellmodule array 630 at around the ground point. In practice, the output ofthe solar cell module array 630 is only approximately balanced becausethe balancing resistors 672 and 673 have high resistance (e.g. about 10MΩ each).

In a typical installation, the solar cell module array 630 would befloating because there would be no resistor 671 and the inverter 650 hasDC-DC isolation between the output of the solar cell module array 630and the AC output to the power grid. The inventors discovered, however,that such an installation will cause harmful polarization of solar cells200. In one embodiment, the positive terminal of the solar cell modulearray 630 is connected to ground by way of a resistor 671. The resistor671 may be a fixed, variable, or electronically controlled resistancewithout detracting from the merits of the present invention. Theresistor 671 biases the solar cell module array 630 closer to thepositive side of its output to prevent positive charge from leaking fromthe front sides of the solar cells 200. In other words, the resistor 671“unbalances” the output of the solar cell module array 630 towardspositive to prevent solar cell polarization. Similarly, if the solarcell polarization is caused by electrons (rather than positive charges)leaking from the front side of solar cells 200, node 617 (instead ofnode 616) may be connected to ground by way of the resistor 671 to biasthe solar cell module array 630 towards its negative output. Theresistor 671 may have a resistance of about ≦ 1/10 of the value of abalancing resistor (i.e. resistor 672 or 673). It is to be noted thatinverter 650 may also be configured such that it unbalances the balancedoutput of the solar cell module array 630 towards positive or negative,depending on the polarity of the leaking charge carrier (i.e. electronsor holes). For example, the value of resistor 672 may be increasedrelative to resistor 673 to unbalance the output of the solar cellmodule array 630 without using the resistor 671.

The resistor 671 may also comprise an electronically controlledresistance. For example, the resistance of the resistor 671 may becontrolled by an electronic circuit by switching in different resistancevalues depending on condition. Such an electronic circuit may havesensors that detect when a lower resistance is needed when the solarcell module array resistance is reduced to ground level, such as whenraining, for example.

In the example of FIG. 7C, each solar cell 200 has an n-type front sidediffusion region. In this case, harmful solar cell polarization occursbecause the solar cells 200 become positively biased relative to ground.To prevent harmful polarization, the highest or near highest potentialof the solar cell module array 630 (node 616 in this case) isaccordingly tied to ground by way of a resistance (e.g. resistor 671).In other embodiments where the solar cells have a p-type front sidediffusion region, harmful polarization may occur when the solar cellsbecome negatively biased relative to ground. In that case, the lowest ornear lowest potential solar cell in the array (e.g. the negative outputterminal of the solar cell module array) may be tied to ground by way ofa resistance to prevent harmful solar cell polarization.

Techniques for preventing harmful solar cell polarization have beendisclosed. While specific embodiments of the present invention have beenprovided, it is to be understood that these embodiments are forillustration purposes and not limiting. Many additional embodiments willbe apparent to persons of ordinary skill in the art reading thisdisclosure.

What is claimed is:
 1. A solar energy system comprising: a series ofsolar cells in a solar cell module array, the series of solar cellshaving a first node and a second node opposite the first node, the firstnode being at a positive potential and connected to a positive outputterminal of the solar cell module array and the second node being at anegative potential relative to the first node and connected to anegative output terminal of the solar cell module array, each solar cellin the series of solar cells having a front side facing the sun duringnormal operation and a backside opposite the front side; a frame of asolar cell module in the solar cell module array, the frame being tiedto ground; and an inverter coupled to the positive output terminal andthe negative output terminal, the inverter being configured to convertdirect current generated by the solar cell module array to alternatingcurrent to be provided to a power grid, the inverter including anisolation circuit that provides isolation from the alternating currentto be provided to the power grid, wherein either the positive outputterminal or the negative output terminal is tied to ground, the positiveoutput terminal being tied to ground when solar cells in the series ofsolar cells have an N-type front side diffusion regions and the negativeoutput terminal being tied to ground when solar cells in the series ofsolar cells have a P-type front side diffusion regions.
 2. The solarenergy system of claim 1 wherein the positive output terminal is tied toground by way of a resistor.